What’s the cost of a new back glass for an iPhone 8 Plus?

Hi, I think that the rear glass will come under other damage. Check the prices for your region but for comparison this is the support page for the United Kingdom. https://support.apple.com/en-gb/iphone/repair/service

You are kind of correct. "Other Damage" is when a phone is covered under an AppleCare+ Plan. If the phone is not covered under AppleCare+, the phone would need to replaced as Out of Warranty because the back glass on iPhone cannot be replaced. This is the link for the US in case the OP is not in the UK --> iPhone Repair - Official Apple Support

As can be seen from the link an iPhone 8 Plus Out of Warranty replacement is $399 in the US.

Apologies, I should have written Other Damage (Out of Warranty)